Pump-probe photons

We have reviewed the EOM-PMA method for the calculation of two-pulse-induced (spontaneous emission, pump-probe, photon echo) and three-pulse-induced (transient grating, photon echo, coherent anti-Stokes-Raman scattering, four-wave-mixing) optical signals. In the EOM-PMA, the interactions of the system with the relevant laser pulses are incorporated into the system Hamiltonian and the driven system dynamics is simulated numerically exactly. 

A. Discrepancy in Absorption Coefficients for Pump-Probe Photons. A key... 

The dynamics of fast processes such as electron and energy transfers and vibrational and electronic deexcitations can be probed by using short-pulsed lasers. The experimental developments that have made possible the direct probing of molecular dissociation steps and other ultrafast processes in real time (in the femtosecond time range) have, in a few cases, been extended to the study of surface phenomena. For instance, two-photon photoemission has been used to study the dynamics of electrons at interfaces [ ]. Vibrational relaxation times have also been measured for a number of modes such as the 0-Fl stretching m silica and the C-0 stretching in carbon monoxide adsorbed on transition metals [ ]. Pump-probe laser experiments such as these are difficult, but the field is still in its infancy, and much is expected in this direction m the near fiitiire. 

Several laser systems have been used in our time-resolved PM measurements. For the ultrafast measurements, a colliding pulse mode-locked (CPM) dye laser was employed [11]. Its characteristic pulsewidth is about 70 fs, however, its wavelength is fixed at 625 nin (or 2.0 cV). For ps measurements at various wavelengths two synchronously pumped dye lasers were used (12], Although their time resolution was not belter than 5 ps, they allowed us to probe in the probe photon energy range from 1.25 cV to 2.2 cV. In addition, a color center laser... 

Figure 10-4. Temporal behavior of the pholoinduccd transmission changes in LPPP alter excitation with a femtosecond pump pulse at 400 nnt. The two curves correspond to probe photon eneigies of 2.48 eV (dotted line) and 1.91 eV (solid line). At 2.48 eV the transmission change is positive due to stimulated emission (SE) while a photoin-dueed absorption (PIA) is observed at 1.91 eV (according to Ref.(24J).

The possible profiles of intensity versus the delay between the pump and probe photons for the several potential processes operative in the mechanism of ionization are shown in Figures 6a, b, and c. Several different laser schemes have been employed to investigate the reaction dynamics pertaining to the formation of protonated ammonia clusters through the A and C states (the latter possibly including the B state as well). The ionization schemes employed in the present study are shown in Figure 7. 

Keywords Cyanine dyes Excited state absorption Polymethine dyes Pump-probe Two-photon absorption Z-scan... 

The term nonlinear optical property refers to an optical property, which can be modified by exposing the material to intense light irradiation. In this section, we focus on the cascaded first- (/ 1 ) and third-order ( / ) susceptibilities describing nonlinear absorption (ESA and 2PA) and nonlinear refraction (n2) processes. Z-scan, pump-probe, and two-photon upconverted fluorescence techniques are among the most used experimental methods for determining optical nonlinearities. 

Photon echo and IR pump-probe experiments can also be performed on neat water, but one needs a very small sample. Fabrication of nano-fluidic Si3Ni4 sample cells have opened up this new and exciting field, and data from these experiments, performed by the Elsaesser and Miller groups, have recently been reported [73, 74]. At room temperature, spectral evolution occurs within 50 fs, and polarization anisotropy decays within 75 fs. At temperatures just about the freezing point, spectral evolution slows down dramatically [74],... 

Fig. 12 Probe photon energy dependence of the time profile for the refiectance change AR/R observed at 180 K and 260 K. The pump photon energy (E//h) was 1.55 eV and the probe photon energy (E//b) was 1.72 and 1.38 eV for the upper and lower panels, respectively. The oscillations in AR/R relate molecular deformation modes [162]...

The pump-probe-detect arrangements for the femtosecond experiments was similar to those described above. When cyclobutanone was pumped with two photons of a X = 307-nm femtosecond pulse, two consecutive C—CO bond cleavages led to the formation of the trimethylene diradical, detected as an easily ionized transient at 42 amu, with buildup and decay times of 120 20 fs. The decay presumably involves isomerizations to cyclopropane and to propylene— structures not ionized by the probe pulse and thus undetected during the experiment. 

Comprehensive experimental investigations of 2PA processes in fluorene derivatives were performed by Hales et al. [53,56-59] with open aperture Z-scan [26], two-photon induced fluorescence [60] and femtosecond white-light continuum pump-probe methods [61]. For degenerate two-photon excitation, the experimental 2PA spectra of symmetrical and asymmetrical fiuorenes are presented in Figs. 15 and 16. These spectra were obtained with the combination of open aperture Z-scan and two-photon fluorescence methods [57]. For centrosymmetric molecules, two-photon transitions from... 

A typical application is the use of the (2 + 1) REMPI scheme for measuring the (v,./) distribution of H2 produced in associative desorption from a surface. When the laser is tuned to a spectroscopic transition between individual quantum states in the X -> E electronic band, resonant two-photon absorption populates the E state and this is subsequently ionized by absorption of another photon. The ion current is proportional to the number in the specific (v,./) quantum state in the ground electronic state that is involved in the spectroscopic transition. Tuning the laser to another spectroscopic feature probes another (v, J) state. Therefore, recording the ion current as the laser is scanned over the electronic band maps out the population distribution of H2(v, J) produced in the associative desorption. Ef of the (v, J) state can also often be simultaneously measured using field - free ion TOF or laser pump - probe TOF detection techniques. The (2 +1) REMPI scheme for detecting H2 is almost independent of the rotational alignment and orientation f(M) of molecules so that only relative populations of the internal states... 

Fig. 2. Photoelectron spectra at different delay times x between pump (263nm) and probe (400nm) pulses for indole(NH3)i+ (a) and mdoleCNH3)4+ (b). The arrows denote the maximum electron energies for one or two probe photon ionization.

The pump-probe scheme that we use is as follows the molecule is initially excited by a pump laser photon (htOj) to the first excited neutral state. The dynamics is then followed with a probe laser by one-photon ionization (hooa). Compared to a free diatomic, solvation by CH3CN brings two new factors come into play. Firstly, the potential energy curves of the diatomic are modified by the presence of the neighboring molecule. Secondly, the fragmentation dynamics of the diatomic is changed as there may be collisions with, and a transfer of energy to, the acetonitrile. 

In a pump-probe configuration, using photodiodes for the detection, significant absorption spectroscopic data are reported in Fig.l. Following the interaction of UV photons with aqueous OH ion, two well defined electronic trajectories are observed in the first 4 ps. 

In the experiment, the transmission intensities for the excited and the dark sample are determined by the number of x-ray photons (/t) recorded on the detector behind the sample, and we typically accumulate for several pump-probe shots. In the absence of external noise sources the accuracy of such a measurement is governed by the shot noise distribution, which is given by Poisson statistics of the transmitted pulse intensity. Indeed, we have demonstrated that we can suppress the majority of electronic noise in experiment, which validates this rather idealistic treatment [13,14]. Applying the error propagation formula to eq. (1) then delivers the experimental noise of the measurement, and we can thus calculate the signal-to-noise ratio S/N as a function of the input parameters. Most important is hereby the sample concentration nsam at the chosen sample thickness d. Via the occasionally very different absorption cross sections in the optical (pump) and the x-ray (probe) domains it will determine the fraction of excited state species as a function of laser fluence. 

In my talk I surveyed recent advances in the methodology and selected 2D-IR spectra of secondary structures. The results promise to provide structurally based kinetic probes for conformational dynamics, sharp tests of anharmonic potential surfaces and novel information regarding the transient and equilibrium vibrational dynamics of peptides. The heterodyned 2D-IR approach has proven useful in determining structures of peptides in solution and the anharmonic nature of the potential surfaces of peptides and secondary structures [1-10], as have polarized photon echo [2,6,10-12] or pump-probe techniques [4,13-16]. 

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